Abstract
The crystal structure of the mineral görgeyite, K~2~SO~4~ · 5 CaSO~4~ · H~2~O, has been determined from visually estimated photographic intensities using the direct methods program MULTAN, and refined by block-diagonal least-squares structure factors to an R value of 0.096. The mineral is monoclinic and contains four formula units in a cell of dimensions a= 17.51(1), b = 6.822(5), c= 18.21(1) Å, β = 113.3° and space group __C__2/c.
Two independent calcium atoms, one of which sits on the diad axis, are nine-fold coordinated by oxygen atoms in a closely similar arrangement, with average Ca–O distances of 2.52 Å in the range 2.40–2.68 Å. A third calcium atom is eight-fold coordinated with a mean Ca–O distance of 2.46 Å in a range of 2.37–2.64 Å.
The potassium atom is also eight-fold coordinated. The three independent sulphate ions are approximately regular with mean S–O distances of 1.472(8), 1.477(27), and 1.470(7) Å respectively but their behaviour in providing oxygen coordination is different for each. The hydrogen atom of the water molecule has not been positively located though it is likely that a hydrogen bond is present.